The experiments described in this proposal are intended to provide information regarding solute and water transport processes in three discrete regions of the mammalian nephron: superficial proximal straight and convoluted tubules, leaky epithelia which absorb relatively large amounts of glomerular filtrate, either isosmotically or near-isosmotically; and the cortical collecting duct, an electrically tight epithelium which absorbs relatively smaller volumes of salt and water, discriminates sharply between solutes and water - and hence dissociates solute and water absorption, and is subject to modulation of water transport by ADH, and of salt transport by aldosterone. The major goals of our experiments are to evaluate: contributions of active and passive transport processes to Na ion absorption in the proximal nephron; mechanisms of rheogenic Na ion transport in the proximal nephron; tubular fluid acidification in the proximal nephron; the diffusion constraints of the extracellular pathway in the proximal nephron; the physical processes by which ADH enhances the rate of water permeation across tubules, including the ways in which aldosterone modulates this process. Our observations may provide a frame of reference for understanding the pathophysiology of body fluid homeostasis disorders such as proximal renal tubular acidosis, natriuretic states, anti-natriuretic states, and nephrogenic diabetes insipidus.